LTC6412 Datasheet, PDF (14/24 Pages) Linear Technology – 800MHz, 31dB Range Analog-Controlled VGA

LTC6412
OPERATION
The LTC6412 employs an interpolated, tapped attenuator
circuit architecture to generate the variable-gain charac-
teristic of the amplifier. The tapped attenuator is fed to a
buffer and output amplifier to complete the differential
signal path shown in the Block Diagram. This circuit
architecture provides good RF input power handling ca-
pability along with a constant output noise and output IP3
characteristic that are desirable for most IF signal chain
applications. The internal control circuitry takes the gain
control signal from the ±V_G terminals and converts this
to an appropriate set of control signals to the attenuator
ladder. The attenuator control circuit ensures that the
linear-in-dB gain response is continuous and monotonic
over the gain range for both slow and fast moving input
control signals while exhibiting very little input impedance
variation over gain. These design considerations result
in a gain-vs-V_G characteristic with a ±0.1dB ripple and
a 0.5μs gain response time that is slower than a similar
digital step attenuator design.
An often overlooked characteristic of an analog-controlled
VGA is upconverted amplitude modulation (AM) noise
from the gain control terminals. The VGA behaves as a
2-quadrant multiplier, so some minimal care is required to
avoid excessive AM sideband noise generation. The table
below demonstrates the effect of the baseline 20nV/√Hz
equivalent input control noise from the LTC6412 circuit
along with the effect of a higher combined input noise due
to a noisy external control circuit.
CONTROL INPUT TOTAL NOISE
VOLTAGE (nV/√Hz)
20
40
70
100
200
PEAK AM NOISE AT 10kHz OFFSET
NEAR MAXIMUM GAIN (dBc/Hz)
–142
–136
–131
–128
–122
The baseline equivalent 20nV/√Hz input noise is seen to
produce worst-case AM sidebands of –142dBc/Hz which is
near the –147dBm/Hz output noise floor at maximum gain
for a nominal 0dBm output signal. An input control noise
voltage less than 80nV/√Hz is generally recommended to
avoid measurable AM sideband noise. While op amp control
circuit output noise voltage is usually below 80nV/√Hz,
some low power DAC outputs exceed 150nV/√Hz. DACs
with output noise in the range of 100nV/√Hz to 150nV/√Hz
can usually be accommodated with a suitable 2:1 or 3:1
resistor divider network on the DAC output to suppress the
noise amplitude by the same ratio. Noisy DACs in excess
of 150nV/√Hz should be avoided if minimal AM noise is
important in the application.
6412f
14